1. Technical Field
The invention is related to environmental field, more specifically it is related to a fertilizer and method for preparation of it. This fertilizer is especially for reducing the content of heavy metal and nitrate in vegetables.
2. Description of Related Art
Vegetables are essential products for human being, and are directly related to their health. With the development of urbanization and industrialization, the soil pollution of vegetable lands, in particular those around cities, is getting worse and worse, which not only influences the yield and quality of vegetables, but also makes the agricultural product export limited by “Green Barriers”. Further, this can be harmful to human through food chains. Within these pollutions, over-standards of heavy metals, nitrides and nitrites are the major problem.
Soil pollution by heavy metal in China is getting worse and is becoming a great threat to the human health. According to statistics, at present, China has nearly 0.15 billion acre of polluted soil which is more than 1/10 of the total agricultural lands, within which 32.5 million in polluted by sewage, 2 million is polluted by solid wastes. Most of these polluted areas are located in developed regions. Every year, the agricultural product decrease caused by soil pollution is up to 12 million tons, which directly leads more than 20 billion RMB of economic loss. Vegetables are not only important economic plants but also essential in people's daily life. Heavy metals could be concentrated in vegetables, and the concentrated metal could be led to human body by food chain, thus is harmful to human beings.
According to current researches which have aroused widely academic interests, by applying silicon fertilizers, the resistance of rice to manganese, iron, cadmium, aluminum and other heavy metals can be improved, thus the absorption and accumulation of those heavy metals in the vegetables can be reduced. However, if the silicon fertilizers are directly applied to soil, the dissolvability thereof is relatively poor and it is easy to be absorbed by soil colloids, which causes poor biological effectiveness. Thus, alleviating effect of silicon fertilizers to metal toxicity is less obvious in field application while compared with pot experiments. For example, though the silicon fertilizers applied to soil can also inhibit the absorption and accumulation of heavy metals such as Pb and Cr etc. in Chinese cabbages, its efficiency is limited; While 500 mg·kg−1 of silicon oxide is applied to 100 mg·kg−1 of cadmium polluted soil, there's only 16.9 percent of increase of the aboveground biomass of Chinese cabbages and 17.8 percent of decrease of the cadmium content in foliage, and the cost of this application per acre is more than 100 RMB. Silica sol has the characteristics of small partical size, large specific surface area, high dispersion, high adhesion, and is easily absorbed by plants. Our previous research proved that foliage fertilization had high utilization efficiency and high absorption, etc., and it is proved that silica sol sprayed onto the rice foliage can improve the resistance of rice to heavy metals and inhibit the transfer of the heavy metals such as Cd and Pb etc. from entering into grain kernels. However, no report was found about inhibiting the vegetables from absorbing and accumulating heavy metals by applying silicon fertilizers onto the foliage. According to our previous parent “Rare Earth Compound Silicon Sol Capable of Inhibiting Heavy Metal Absorption of Rice” (ZL200610036994.8), the compound silicon sol is specially developed for rice, and the foliage fertilization has a good result in inhibiting the rice from absorbing and accumulating the heavy metals; but most of the vegetables are dicotyledon, the silicon absorption and utilization thereof is greatly different from the rice which is the monocotyledon, so Technical scheme using silicon sol for inhibiting the vegetables from absorbing the heavy metals and its foliage fertilization technology thereof are greatly different from those of the rice. Currently, no special foliage fertilizer for inhibiting vegetables from absorbing the heavy metals has been found yet.
Most of the soil pollution is combined pollution. Besides of heavy metals, nitrides also are a common pollutant. Because of abusive use of chemical fertilizers, especially the over use of nitrogen fertilizers (urea and ammonium sulfate etc.), vegetables are seriously polluted by nitrides. Nitrides can be easily enriched in vegetables. Generally, levels of nitride accumulation in different vegetables ranked as follow: leaf vegetables>root vegetables>bulb vegetables>melon and fruits>beans>solanaceous fruit vegetables.
Leaf vegetables' nitride pollution problem are serious in some regions in China. For example, the nitride content of spinaches, Chinese Cabbages, plump white Chinese cabbages, Ipomoea aquatica, and Brassica parachinensis sampled in a market in Shenzhen are all more than 3,100 mg/kg, which means the vegetables are seriously polluted by nitrides. After nitrogen fertilizer or other nitride fertilizer is applied, the nitride will be enriched in the vegetables. Though the toxicity of nitrides is not high, but while nitrides entered human body it could convert into nitrites, which have high carcinogenicity to the human body. Once entering the blood, the nitrite can be combined with hemoglobin to generate hemoglobin, which makes the hemoglobin unable to be combined with or separated from oxygen, thus lose the capability of carrying oxygen. More seriously it can cause respiratory center paralysis, asphyxia and death. On the other hand, nitrite is the precursor of nitrosamine which is harmful to human for its carcinogenicity, teratogenecity and Mutagen, and further induces canceration of the digestion system.
With the rapid development of industry and agriculture and with the abusive use of pesticides and chemical fertilizers, the problem of heavy metal-nitride combined pollution of farm lands become more and more serious. To pursue high yields and stable yield of vegetables, a great amount of pesticides and chemical fertilizers were continuously used for years during vegetable cultivation and therefore causes a great amount of nitrides and heavy metals to accumulate in the soil. At present, Solutions to treating the heavy metal soil pollution mainly comprise engineering treatment, biological treatment, agricultural treatment and chemical treatment etc. The engineering treatment is mainly applied to seriously polluted soil, while biological methods are mainly applied to the lightly polluted soil; however, these methods usually need huge investment but with limited effect while applied in widely polluted soil treatment. In vegetable nitride control, the reduction of use of nitrogen fertilizer is a main means which leads to vegetable production decrease to a certain extent. Usually, farms pay more attention on yield when there is a conflict between yield and quality. Though accumulation of nitride in vegetables could be controlled by choosing vegetable types, adjusting the conditions of water, illumination and temperature etc. during the growth of the vegetables, but its effect is unsatisfactory. More over, among the vegetable pollution control measures, those for preventing and controlling heavy metal pollution and nitride pollution are separated. Solution to controlling the heavy metal and nitride combined pollution has not been found yet.